240 CONFEKENCE ON PALEOZOIC PALEOGEOGRAPHY 



are quoted from Prof. Johan Hjort's article on "The Michael Sars North 

 Atlantic Deep-Sea Expedition :" ^ 



. . . "Now, if we calculate the depth to which the rays of the sun pene- 

 trate, after passing through the same distance in the water, assuming always 

 that the rays are direct, and that the rate of obsorption is the same, we find 

 that the rays will have passed through the same distance to reach a depth of 

 500 meters in 50 degrees north latitude that they will pass through to reach 

 650 meters in 33 degrees north latitude, or 300 meters in 67 degrees north 

 latitude. 



"However, the transparency of the water varies greatly in different regions. 

 If w^e take the results of previous observations during different expeditions, we 

 may set down the visible depth in the open sea as being, roughly, 50 meters in 

 33 degrees north latitude, 40 meters in 50 degrees north latitude, and 25 meters 

 at the outside in the Norwegian Sea in 67 degrees north latitude. Taking this 

 into consideration, we find that there will be the same intensity from the 

 rectilinear rays — 



"In 33 degrees north latitude, at about 800 meters' depth. 



"In 50 degrees north latitude, at about 500 meters' depth. 



"In 67 degrees north latitude, at about 200 meters' depth." 



"During the Atlantic cruise of the Michael Sars we undertook a series of 

 measurements of the intensity of light with a photometer constructed by Dr. 

 Helland-Hansen ; to determine the intensity of the different color rays, Dr. 

 Helland-Hansen made use of panchromatic plates and- gelatine color-filters. 

 The observation south and west of the Azores (that is to say, at the southern 

 stations) showed that the rays of light strongly affected the plate at a depth 

 of 100 meters. The red rays were weakest here, while the blue and ultra-violet 

 rays were strongest. At a depth of 500 meters the blue and ultra-violet rays 

 were still distinctly visible, and at a depth of 1,000 meters the ultra-violet rays 

 were yet perceptible. In 1,700 meters, however, there was not the faintest 

 trace of Ijght, even after the plates had been exposed for two hours in broad 

 daylight." 



A natural experiment at the Tortugas shows the effect of light on the 

 habitat of shallow-water species. The government wharf at Fort Jef- 

 ferson is supported by iron piles coated with cement. On all the periph- 

 eral piles (on both ends and on both the landward and seaward sides of 

 the wharf) there are many corals, while those in the permanently shaded 

 area bear none at all. Species of reef corals placed in a light-proof live- 

 car die after the light has been excluded for several weeks. Strong light 

 is one of the essentials for the life of recent species of reef corals.* 



3 This quotation is from a proof copy kindly loaned by Sir John Murray. The article, 

 a lecture before the Royal Geographical Society, appears in the Geographical Journal, 

 1911 ; but as I have not, on June 22, seen the issue of the Magazine containing it, I 

 can not give the page reference. 



*The presence of commensal algae (Zoanthoxellse) is here noted, but a discussion of 

 them and their functions would be too great a diversion. 



