  We present the spectral evolution, light curve, and corresponding
interpretation for the "normal-bright" Type Ia Supernova 2005cg discovered by
ROTSE-IIIc. The host is a low-luminosity (M_r = -16.75), blue galaxy with
strong indications of active star formation and an environment similar to that
expected for SNe Ia at high redshifts. Early-time (t ~ -10 days) optical
spectra obtained with the HET reveal an asymmetric, triangular-shaped Si II
absorption feature at about 6100 \AA with a sharp transition to the continuum
at a blue shift of about 24,000 km s^-1. By 4 days before maximum, the Si II
absorption feature becomes symmetric with smoothly curved sides. Similar Si II
profile evolution has previously been observed in other supernovae, and is
predicted by some explosion models, but its significance has not been fully
recognized. Although the spectra predicted by pure deflagration and delayed
detonation models are similar near maximum light, they predict qualitatively
different chemical abundances in the outer layers and thus give qualitatively
different spectra at the earliest phases. The Si line observed in SN 2005cg at
early times requires the presence of burning products at high velocities and
the triangular shape is likely to be formed in an extended region of slowly
declining Si abundance that characterizes delayed detonation models. The
spectra show a high-velocity Ca II IR feature that coincides in velocity space
with the Si II cutoff. This supports the interpretation that the Ca II is
formed when the outer layers of the SN ejecta sweep up about 5 x 10^-3 M_sun of
material within the progenitor system. (Abridged)
