Correspondence — Prof. J. W. Dawson. 141 



forms, and not as organisms, we must be struck by the utter absence 

 of all crystalline forms, especially in those very minerals which 

 always, and occasionally also in meteorites, appear in a crystallized 

 form . 



"Further, the external forms, and consequently the outlines of 

 the enclosures, harmonize so perfectly with their internal form and 

 structure, that we cannot entertain the idea that these enclosures 

 had been rolled about and ground down before they became finally 

 imbedded in the chondrites. 



" The idea of an aggregate of crystals, if still looked upon with 

 favour, would be contradicted by the fact that the enclosed balls 

 or globes are all constructed excentrically, whereas all terrestrial 

 crystallites are formed concentrically." 



COI^S;:ESI^OI^^IDJE]I^^O:E3. 



STEOMATOPOEA AND CAUNOPjOEA. 

 Sir, — In the August (1880) Number of the Geological Magazine, 

 which owing to some error of transmission has come to hand only a 

 few days ago, I observe an interesting paper by Dr. F. Eoemer, on the 

 relation sometimes observed between the growth of Stromatoporce 

 and tubular corals ; and which in some formations in this country 

 is so common as to have been regarded as a sort of " commensalism." 

 I have referred to these cases in my paper on Stromatojporidce in the 

 Journal of the Geological Society for February, 1879, as well as to 

 other perforations, probably due to the opeiations of some boring 

 animal. While, however, some specimens of these kinds may have 

 been referred to the genus Caunopora, it would be unfortunate if 

 paleontologists should suppose that all the fossils of that genus are 

 of the character in question. It will be seen by reference to the 

 paper above cited, that such CaunoporcB as my C. Hudsonica, as well as 

 C. incrustans and C planulatum, Hall, not only have vertical canals 

 which are essential parts of their structure, but that these canals 

 send forth radiating tubes into the substance of the thickened laminee. 

 Of the Stromatoporidce with such vertical canals there are two types, 

 which I have referred respectively to the genera Caunopora of 

 Phillips, and Coenostroma of Winchell : the former having single 

 vertical tubes, the latter groups of such tubes. In America both 

 genera begin in the Niagara formation and extend upward to the 

 Chemung, or from the lower part of the Upper Silurian to the 

 upper part of the Erian or Devonian. J. W. Dawson. 



McGill's College, Montreal. 



ON CERTAIN CASES OF THE OUTCEOP OF STEATA. 



Sir, — As the Eev. 0. Fisher's allusions to Spherical Trigonometry 

 in your January Number may sadly perplex many accomplished 

 geologists, who have not made a special study of higher mathe- 

 matics, I venture to enclose a simplified explanation of his results. 



First, as regards the delineation of cylindrical surfaces exposed 



