ADDITIONS TO THE DICTYOSPONGIDiE. 23 



Sir William Dawson remarked, concerning the subject of the latter paper: We 

 in Canada, have now got as far back as the Siluro-Cambrian and Cambrian systems 

 in the history of the Dictyospongidas, several species of Protospongia and Gyathospongia 

 having been obtained from the Quebec group. We have thus got a little further 

 back in the series than you have in the United States. We have also another genus 

 of the same group, which Hinde describes under the name of Acanthodictya. Twelve 

 species in all are described in a paper on Fossil Sponges of the Quebec group, now in 

 the press for the transactions of the Royal Society of Canada. Another point to 

 which I would refer relates to the opinions entertained on the animal nature of these 

 curious forms. Many years ago Professor Hall was kind enough to send me speci- 

 mens of them. I had grave doubts about what they were, but could not refuse to 

 call them plants, because there were no traces of spicules upon them, and there seemed 

 to be evidences of an external membrane ; and therefore I thought they could scarcely 

 be sponges. They were then named Dictyophyton. A little later the intercross- 

 ing spicules were found, and I was shown a specimen of them in the Natural History 

 Museum of New York; and I was then very thankful to be able to say I had been 

 mistaken, and that we could no longer regard them as plants. We are, I think, very 

 much indebted to the President for the work he has bestowed upon these interest- 

 ing organisms, which constitute so marked an instance of a permanent animal type, 

 culminating in a very early period. 



The Society then took a recess until 2 p. m. 



At the appointed hour the Society reassembled, Vice-President Alex. 

 Winchell occupying the chair. 



The following communication was presented : 



THE STRENGTH OF THE EARTH'S CRUST. 

 BY G. K. GILBERT. 



[Abstract.'] 



The term crust is. here used to indicate the outside part of the earth, without refer- 

 ence to the question whether it differs in constitution from the interior. 



Conceive a large tank of paraffine with level surface. If a hole be dug in this and 

 the material piled in a heap at one side, the permanence of hole or heap will depend 

 on its magnitude. Beyond a certain limit, further excavation and heaping will be 

 completely compensated by the flow of the material. Substitute for paraffine the 

 material of the earth's crust, and the same result will follow, but the limiting size of 

 the hole or heap will be different, because the strength of the material is not the 

 same. Assuming the earth to be homogeneous, the greatest possible stable promi- 

 nence or depression is a measure of the strength of its material. 



It is not believed that the earth is homogeneous, and with reference to the outer 

 portion of the crust it is known that it is not composed of homogeneous shells. There 

 is observational basis for the theory that the matter composing and lying beneath 

 continents is lighter than the matter composing and lying beneath ocean beds, and 

 many students of terrestrial physics entertain the theory that unit columns extending 

 from the surface downward have everywhere the same weight, the height of each 



