368 TRANS. ST. LOUIS ACAD. SCIENCE. 



Prof. Dana, in his Manual of Geology^ '874, speaks of the 

 rocks at Pilot Knob, Mo., as Laurentian. 



Our best authorities on the Archaean rocks are Dana and Hunt ; 

 they have carefully studied them and are high authorities, yet we 

 know that in many important points of geology they do not agree. 

 Their opinions are greatly to be respected. We honor them for 

 their life-long devotion to science. We could wish that they could 

 visit every place where porphyry or granite is found in Missouri. 

 Prof. Hunt has seen the porphyries at Iron Mountain and Pilot 

 Knob and near by these places. We do not know to what extent 

 nor how far from those places he has seen them. 



Prof. Pumpelly studied the porphyries and iron ores very care- 

 fully near and at those points, but I scarcely think he has seen 

 sufficient to form a decided opinion of the age of the rocks of the 

 whole region ; for, in a note on page 3 of Mo. Geol. Rep. 1S72, 

 he states that there are no hornblendic dykes in the porphyry. 

 Now, I know of a dyke of hornblendic rock, 6 feet wide, standing 

 up like a wall and in a porphyry region, 14 miles south of Pilot 

 Knob, on south side of Buck Mountain. Greenstone is frequently 

 found in both Iron and Madifeon Counties, Mo., associated with 

 porphyry; and, although more often found in loose,. tumbled 

 masses, it occurs along a certain course, so as to leave no doubt of 

 its being from dykes protruding through porphyry. In the Geo- 

 logical Report for 1874 (Madison Co. Rep.) several greenstone 

 and dolerite dykes are mentioned, some protruding through gran- 

 ite, others through porphyry. 



Not having seen the rocks on the Atlantic coast referred to by 

 Prof. Hunt, I am unable fully to compare ours, but I think it 

 highly probable that our granites and porphyries may be of dif- 

 ferent age ; the granites may be older than the porphyries. Very 

 careful geological study of these districts of Missouri may result 

 in important deductions very useful to science. 



In these rocks we have no fossils to guide us, and we can only 

 be guided by their lithological appearance, or mineral contents. 

 Prof. Hunt's description of the eastern porphyries would answer 

 for many of ours. Ours are divided mainly into two classes, one 

 an exceedingly fine-grained jaspery rock, and the other a coarser 

 one, which in texture resembles a granite. The finer grained 

 ones are either black, or with some shade of red ; in fiict, even the 



