

XIII.] GEOGNOSY OF THE APPALACHIANS. 247 



steatites, hornblendic, dioritic, chloritic, and micaceous schists 

 (often garnet-bearing), together with a band of argillite, afford- 

 ing roofing-slates. With this great series are associated chromic 

 and titanic iron, and ores of nickel and copper. Veins of 

 albite with corundum also intersect this series near Unionville. 

 We are repeatedly assured by Eogers that these rocks so much 

 resemble the underlying hypozoic gneiss, as to be readily con- 

 founded with them; and when compared with the latter, as 

 displayed in the southern district, it is difficult to believe that 

 we have in this so-called azoic or metamorphic series of the 

 Montgomery and Chester valleys anything else than a repeti- 

 tion of these same crystalline schists which have been described 

 along their southern boundary, representing the Green Moun- 

 tain and the White Mountain series. We thus avoid the 

 difficulty of supposing that we have in this region two sets of 

 serpentine rocks, end two of mica-schists, lithologically similar, 

 but of widely different ages, a conclusion highly improbable. 

 It should be said that Eogers, in accordance with the notions 

 then generally received, looked upon serpentine as an eruptive 

 rock, which had altered the adjacent strata, converting the 

 mica-schists into steatitic and chloritic rocks. 



This so-called azoic series, according to Rogers, underlies the 

 auroral limestone of Pennsylvania, thus apparently occupying 

 the horizon of the primal paleozoic division. We find, how- 

 ever, in his report on the geology of the* State, no satisfactory 

 evidence of the identity of the two series of crystalline rocks. 

 On the contrary, a very different conclusion would seem to 

 follow from certain facts there detailed. The azoic or so-called 

 metamorphic primal strata are said to have a very uniform 

 nearly vertical dip, or with high angles to the southward, while 

 the micaceous and gneissic strata of the northern subdivision 

 of the southern district of so-called hypozoic rocks, limiting 

 these last to the south, present either minute local contortions 

 or wide gentle undulations, with comparatively moderate dips, 

 for the most part to the northward.* From this, I think, 

 we may infer that the nearly vertical strata must be, in truth, 

 * Rogers, Geology of Pennsylvania, I. pp. 69 - 74, and 154 - 158. 



