376 A SLTLOR SAD SICA GRS: 
ally and chemically the two kinds are practically identical. It is quite 
manifest that the two phases gradually merge into each other, and the 
porphyry is to be regarded as the surface facies of the coarse-grained 
rock. 
Heretofore no explanation has been advanced regarding the 
peculiarities in the geographic distribution of the granitic and por- 
phyritic masses. It is the purpose of the present note to ascribe the 
surface distribution, as now existing, to certain stages in the physio- 
graphic development of the region. In its main features the crystalline 
district of Missouri is a semi-alpine country. The prominent solitary 
peaks are irregularly distributed, and form what has been called the St. 
Francois Mountains. This group of hills constitutes the eastern end 
of the crest of the Ozark uplift. The extremes of altitude are about 
500 and 1800 feet above tide level. 
Most prominent ainong the physiographic features presented are 
two plains standing at different levels. ‘The first is a deeply incised 
constructional surface—the Tertiary peneplain; and the second is a 
moderately dissected plain lying at a lower level—the Farmington 
lowland. The great Tertiary peneplain forms the general surface of 
the Ozark uplift. In no part of this raised region, unless it be in the 
St. Francois district, does any portion of the pre-Tertiary surface pro- 
ject above the broad constructional plain, and even here the nearly 
uniform height to which the numberless peaks rise would indicate that. 
they also were practically obliterated in Tertiary times, at least as 
prominent surface features. The Farmington lowland is formed by a 
broad belt of rather even surface which cuts across the eastern end of 
the crest. Its general elevation is about tooo feet above mean tide, a 
level which is 700 to 800 feet below the horizon of the great peneplain. — 
The lowland is manifestly a plain of denudation. It is the product of 
a former cycle whose work was interrupted before completion. In 
point of time this cycle was a later one than that represented by the 
Tertiary peneplain, and immediately preceded the present one. 
In the areal distribution the principal granite field is confined to 
the northeastern part of the crystalline district and occupies about one- 
fifth of the entire area. The particular fact to be noted regarding it 
is that it lies entirely within the lowland plain of denudation. The 
inference is clear that at this point in the Ozark uplift the porphyritic 
surface facies of the granite has been removed through erosion in part 
perhaps in pre-Cambrian times but largely during a more recent period. 
