338 GEOLOGY OF THE SECOND DISTRICT. 



In its primary system, St. Lawrence is unlike the other counties. The proportion of pri- 

 mary rocks to the sedimentary does not differ materially from equality. All the towns occu- 

 pying the northwest angle belong to the latter class, and all those of the opposite and sontlieast 

 angle are primary. In the central portion of the county, on a line parallel with the St. Law- 

 rence river, both classes are intermixed, and it is on this line that we find the most confusion 

 and trouble in defining distinctly the outlines. The line of separation is extremely crooked 

 and irregular ; in fact, small patches of the sedimentary rocks are found far up on the pri- 

 mary, and cut off from any connection with the system to which they belong. And it is here 

 that we find an area of a few square feet of the potsdam sandstone, three or four feet thick, 

 remaining upon the primary ; the whole bed, except this little patch, having been broken up 

 and washed away, while this adhered obstinately to the mass below ; but it has been almost 

 obliterated, by the grinding of gravel and boulders which have passed over it. Owing to facts 

 of this kind, I have not attempted to be exact in marking the divisions of the rocks, unless 

 some fact of more than ordinary consequence rendered it important to estabhsh for a limited 

 space an accurate boundary line. 



The granite of St. Lawrence possesses the following characters : 



1. The reddish and groy granite at Alexandria, associated with ordinary gneiss and hornblende, It is the common 



granite of New-England ; contains imjierfect crystals of Mack tourmaline, and imperfect garnets. 



2. A coarse grey or white granite, containing albite. It contains also perfect crystals of feldspar, pyroxene, smoky quartz, 



etc.; but it is better known as being always associated with primary limestone. 



The first of these forms of granite, as just remarked, does not differ at all from the granite 

 of New-England ; and this statement holds good, not only so far as its composition is con- 

 cerned, but also in the minerals which it contains ; that is, if it contains any, they will be 

 found without exception of the same kind and character. 



The second variety is whiter than the former ; is liable to decomposition and disintegration ; 

 and is coarse grained, often containing (besides large masses of feldspar, quartz, and plates 

 of mica, which constitute the rock itself) imbedded crystals, sometimes arranged in veins, 

 and at others without any order of this kind. This last variety may be said to occur in three 

 modes, though I do not attach much importance to the distinction : 



1. In large irregular beds, or protruding masses. 



2. In the form of veins, branching irregularly into the adjacent rock. 



3. In overlying masses, analogous to the overflowing of lava currents, or of greenstone. 



The constant associate of these varieties is primary limestone ; and as it regards superpo- 

 sition, the limestone is as constantly beneath, as the granite. In some places a mixture pre- 

 vails, forming a perishable rock, but one in which may be found interesting mineral species. 



Having given, in the early part of this report, a full account of this variety of granite and 

 its associate, primary limestone, it seems to be unnecessary to repeat the facts again. I shall 

 therefore only give those which are mostly additional, and which may have a local interest, 

 and require a detail more minute than was given in the general account of these rocks. 



